Process for the synthesis of 1,3-diols

ABSTRACT

An improved process for the preparation of cis-1,3-diols is described where a beta hydroxy ketone is treated with a trialkylborane or dialkylalkoxyborane or a mixture of a trialkylborane and a dialkylalkoxyborane followed by recovery and reuse of the alkylborane species to convert additional beta hydroxy ketone to the cis-1,3-diol.

This application is a 371 of PCT/4598/25493, filed Dec. 2, 1998 and claims priority to Provisional Application 60/068,193, filed Dec. 19, 1997.

BACKGROUND OF THE INVENTION

The present invention relates to a process for preparing cis-1,3-diols. More particularly, the present invention relates to the use and subsequent recovery and reuse of a trialkylborane or dialkylalkoxyborane or a mixture of a trialkylborane and a dialkylalkoxyborane in the reduction of a beta-hydroxy ketone to obtain a cis-1,3-diol. Additionally, the present invention relates to the use of a synergistic combination of a trialkylborane and a dialkylalkoxyborane in the reduction of a beta-hydroxy ketone to obtain a cis-1,3-diol.

The use of trialkylboranes or dialkylalkoxyboranes in the stereoselective reduction of 1,3-keto alcohols to the corresponding syn-1,3-diols has been widely described in the literature. This method has given high stereoselectivity without using extraordinarily difficult conditions (Brower P. L., Butler D. E., Deering C. F., Le T. V., Millar A., Nanninga T. N., and Roth B., Tetrahedron Lett., 1992;33:2279; Narasaka K., and Pai F. C., Tetrahedron, 1984;40:2233: Chen K. M., Hardtmann G. E., Prasad K., Repic O., and Shapiro M. J., Tetrahedron Lett., 1987;28:155; Chen K. M., Gunderson K. G., Hardtmann G. E., Prasad K., Repic O., and Shapiro M. J., Chem. Lett., 1987:1923). There seems to be general acceptance of the formation of a borate ester from either the trialkyl or dialkylalkoxyboranes which is said to form a cyclic chelate (Narasaka K. and Pai F. C., Tetrahedron, 1984;40:2233; Chen K. M., Hardtmann G. E., Prasad K., Repic O., and Shapiro M. J., Tetrahedron Lett., 1987;28:155; Chen K. M., Gunderson K. G., Hardtmann G. E., Prasad K., Repic O., and Shapiro M. J., Chem. Lett., 1987:1923; see for example Paterson I., Cumming J. G., and Smith J. D., Tetrahedron Lett., 1994;35:3405). Axial delivery of a hydride to the complex then leads predominately to the syn-product which can be hydrolyzed to the diol. The diols are valued as intermediates for the preparation of, for example, HMG-CoA reductase inhibitors which are useful hypolipidemic and hypocholesterolemic agents. This is a widely used method of preparation of such agents (U.S. Pat. Nos. 4,645,854, 5,354.772, 5,155,251, and 4,970.313).

Many procedures in the literature, describe the work-up of the reaction with hydrogen peroxide (U.S. Pat. Nos. 4,645.854 and 4,970,313). This results in the destruction of active alkylborane species. Some procedures describe the repeated distillation with methanol and an acid (U.S. Pat. Nos. 5,354,772 and 5,155,251). This also dilutes and eventually destroys the active alkylborane species. We have surprisingly and unexpectedly found that by performing the reduction and workup with a minimal amount of acid, and keeping the distillate streams separated, that the initial distillate can be recovered and reused to obtain very good selectivity in subsequent reductions.

Thus, the present process offers significant advantages over the prior art processes. For example, the cost of additional alkylborane is eliminated for each batch in which the distillate stream is recycled. Additionally, since alkylboranes are hazardous, they must be destroyed prior to being disposed. The present process minimizes this expensive and time-consuming procedure. Moreover, it is especially surprising that very good selectivity in the reductions is obtained using recovered alkylboranes.

Finally, we have also surprisingly and unexpectedly found that a combination of a trialkylborane and a dialkylalkoxyborane is synergistic in selectively reducing a beta-hydroxy ketone to obtain a cis-1,3-diol.

SUMMARY OF THE INVENTION

Accordingly, a first aspect of the present invention is a process for the preparation of a compound of Formula I

wherein R is alkyl,

NC—CH₂—,

PG—O—CH₂— wherein PG is a protecting group,

R¹ is alkyl or

—CH₂—CO₂R⁶ wherein R⁶ is alkyl;

which comprises:

Step (a) treating a compound of Formula II

or a compound of Formula III

wherein R and R¹ are as defined above with a trialkylborane or dialkylalkoxyborane or a mixture of a trialkylborane and a dialkylalkoxyborane in a solvent;

Step (b) adding an alkali metal hydride at about −110° C. to about —50° C.;

Step (c) concentrating the reaction by distillation to afford a compound of Formula I and a distillate containing alkylborane species; and

Step (d) treating additional compound of Formula II or III with the distillate from Step (c) containing recovered alklylborane species and repeating Steps (b) and (c) as desired to afford additional compound of Formula I.

A second aspect of the present invention is a process for the preparation of a compound of Formula I

wherein R is alkyl,

NC—CH₂—,

PG—O—CH₂— wherein PG is a protecting group,

R¹ is alkyl, or

—CH₂—CO₂R⁶ wherein R⁶ is alkyl;

which comprises:

Step (a) treating a compound of Formula II

or a compound of Formula III

wherein R and R¹ are as defined above with a syncrgistic combination of a trialkylborane and a dialkylalkoxyborane in a solvent; and

Step (b) adding an alkali metal hydride at about −110° C. to about −50° C. to afford a compound of Formula I.

A third aspect of the present invention is a synergistic combination comprising a trialkylborane and a dialkylalkoxyborane.

DETAILED DESCRIPTION OF THE INVENTION

In this invention the term “alkyl” means a straight or branched hydrocarbon radical having from 1 to 10 carbon atoms and includes, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, secondary-butyl, isobutyl, tertiary butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, and the like.

“PG” means a protecting group used for protecting an alcohol moiety such as, for example, benzyl and the like. Additional examples of protecting groups for an alcohol moiety are disclosed at Chapter 2 in Greene T. W., “Protective Groups in Organic Synthesis”, John Wiley & Sons, Inc., 1981.

“Alkali metal” is a metal in Group IA of the periodic table and includes, for example, lithium, sodium, potassium, and the like.

“Alkaline-earth metal” is a metal in Group IIA of the periodic table and includes, for example, calcium, barium. strontium, and the like.

“Alkali metal hydride” includes, for example, sodium borohydride, zinc borohydride, lithium borohydride, lithium aluminum hydride. and the like.

“Alkylborane species” means a mono, di- or trialkylborane where the mono or dialkylborane is further substituted by hydrido or alkoxy as defined hereinafter or a dimeric alkylborane species.

“Alkoxy” means 0-alkyl as defined above for alkyl.

As previously described, the compounds of Formula I are either useful as inhibitors of the enzyme 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG CoA reductase) or are useful as intermediates to prepare HMG CoA reductase inhibitors.

Thus, the present process can be used to prepare various HMG CoA reductase inhibitors containing a cis-1,3-diol moiety. For example, atorvastatin disclosed and described in U.S. Pat Nos. 4.681,893 and 5,273,995; fluvastatin disclosed and described in U.S. Pat No. 5,354,772; bervastatin disclosed and described in U.S. Pat No. 5,082,859; cerivastatin disclosed and described in U.S. Pat. No. 5,177,080; NK-LO4 disclosed and described in U.S. Pat No. 5,011,930; dalvastatin disclosed and described in U.S. Pat No. 4,863,957; glenvastatin disclosed and described in U.S. Pat No. 4,925,852; erythro-7-[5-(2,2-dimethyl-butyryloxymethyl)-4-(4-fluorophenyl)-2,6-diisopropylpyridin-3-yl]-3,5-dihydroxy-6(E)-heptenoic methyl ester disclosed and described in U.S. Pat. Nos. 5,006,530, 5,169,857, and 5,401,746; 7,7′-[2-(dimethylamino)-4-(4-fluorophenyl)-6-isopropylpyridine-3,5-diyl]bis [erythro-(E)-3,5-dihydroxy-6-heptenoic acid methyl ester disclosed and discribed in U.S. Pat No. 5,145,857; 7-[6-cyclopropyl-4-(4-fluorophenyl)-2-(4-methoxyphenyl)pyrimidin-5-yl]-3,5-dihydroxy-6(E)-heptenoic acid sodium salt disclosed and described in U.S. Pat. No. 5,026,708; (E)-7-[4-(4-fluorophenyl)-2-isopropylquinolin-3-yl]-3,5-dihydroxy-6-heptenoic acid δ-lactone disclosed and described in U.S. Pat. Nos. 5,011,930, 5,102,888, and 5,185,328; trans-(E)-6-[2-[2-(4-fluoro-3-methylphenyl)-6,6-dimethyl-4-(N-phenyl-carbamoyloxy)-1-cycloheyenyl]vinyl]-4-hydroxytetrahydropyran-2-one disclosed and described in U.S. Pat. No. 5,001,144; erythro-(E)-7-[2-(4-fluoro-3-methylphenyl)-4,4,6,6-tetramethyl-1-cyclohexen-1-yl]-3,5-dihydroxy-6-heptenoic acid sodium salt disclosed and described in U.S. Pat No. 4,863,957; (E)-trans-6-[2-[2-(4-fluoro-3,5-dimethylphenyl)-4-hydroxy-6,6-dimethyl-1-cyclohexenyl]vinyl]-4-hydroxytetrahydropyran-2-one disclosed and described in U.S. Pat No. 4,900,754; ethyl E-(3R,5S)-7-[4′-fluoro-3,3′,5-trimethyl(1,1′)biphenyl-2-yl]-3,5-dihydroxy-6-heptenoate disclosed and described in U.S. Pat No. 4,567,289; 3(R), 5(S)-dihydroxy-7-[4-(4-fluorophenyl)-1-isopropyl-3-phenyl-1H-pyrazol-5-yl]hept-6(E)-enoic acid disclosed and described in U.S. Pat No. 4,613,610; and (3R,5S)-BMY-21950 disclosed and described in U.S. Pat No. 4,897,490 can be obtained using the present process. All of the aforementioned U.S. patents are herein incorporated by reference.

The process of the present invention in its first aspect is an improved, economical, and commercially feasible method for preparing a compound of Formula 1. The process of the present invention in its first aspect is outlined in Scheme 1.

Thus, a compound of Formula II

wherein R is alkyl,

NC—CH₂—,

PG—O—CH₂— wherein PG is a protecting group,

R¹ is alkyl, or

—CH₂—CO₂R⁶ wherein R⁶ is alkyl;

or a compound of Formula III wherein R and R¹ are as defined above is treated with about 0.1 to about 2.0 molecular equivalents of a trialkylboranc such as, for example, triethylborane, tripropylborane, tri n-butylborane, tri sec-butylborane and the like or a dialkylalkoxyborane such as, for example, dimethylnmethoxyborane, dimethylethoxyborane, dimethylisopropoxyborane. diethylmethoxyborane, diethylethoxyborane, diethylisopropoxyborane, diisopropylmethoxyboranc, diisopropylethoxyborane, diisopropylisopropoxyborane, and the like or a mixture of a trialkylborane and a dialkylalkoxyborane as described previously, followed by the stereoselective reduction with about 1 molecular equivalent of an alkali metal hydride such as, for example, sodium borohydride, zinc borohydride, lithium borohydride, lithium aluminum hydride, and the like; in a solvent such as a hydrocarbon, for example hexane, toluene, cyclohexane and the like; an alkanol, for example methanol, ethanol, isopropanol and the like; or an ether, for example diethyl ether, tetrahydrofuran, 1,2-dimethoxyethane, triglyme (triethylene glycol dimethyl ether) and the like, or mixtures thereof at a temperature of about −110° C. to about −50° C. to afford after concentration by distillation a compound of Formula I. Additional compound of Formula II or III is subsequently treated with the distillate obtained by vacuum distillation from the first run followed by stereoselective reduction carried out as described above to afford a second batch of a compound of Formula I. Thus, recovered quantities of the alkylborane species can be used to convert additional quantities of a compound of Formula II or III to a compound of Formula I. This procedure using recovered alkylborane species can be repeated as desired to obtain additional quantities of a compound of Formula I.

Preferably, the reaction is carried out with about 1.2 to 0.8 molecular equivalents of triethylborane or diethylmethoxyborane or a mixture of a trialkylborane and a dialkylalkoxyborane as described previously in a solvent, preferably a mixture of tetrahydrofuran and methanol, at a ratio of about 8 volumes of tetrahydrofuran to one volume of methanol. This is followed by the addition of about one molecular equivalent of sodium borohydride at about −110° C. to about −50° C., preferably at −80° C., followed by stirring for about 30 minutes to about 3 hours. Under these preferred conditions, greater than 90% of a compound of Formula I is produced in the desired stereochemical conformation.

Preferably, the present process is used to prepare [R-(R*,R*)]-1,1-dimethylethyl 6-cyano-3,5-dihydroxyhexanoate which is used as an intermediate to prepare atorvastatin. Compounds of Formula II or III are either known or capable of being prepared by methods known in the art.

The process of the present invention in its second aspect is an improved, economical, and commercially feasible method for preparing a compound of Formula I as previously outlined in Scheme 1.

In this aspect of the invention, applicants have found that a combination of a trialkylborane and a dialkylalkoxyborane surprisingly and unexpectedly is synergistic in selectively affording the desired cis-1,3-diol over the undesired trans-1,3-diol compared to the use of either a trialkylborane or a dialkylalkoxyborane alone. The synergistic combination comprises about 1% to 99% by weight of a trialkylborane and about 99% to 1% by weight of a dialkylalkoxyborane; preferably, a combination of about 90% by weight of a trialkylborane and 10% by weight of a dialkylalkoxyborane. This synergistic combination is of particular advantage since it does not require a stir time of the alkylborane species with the hydroxyketone at ambient temperature before reduction. The conditions and solvents for carrying out the reaction with a synergistic combination of a trialkylborane and a dialkylborane are as previously described above.

Thus, for example, in the preparation of [R-(R*,R*)]1,1-dimethylethyl 6-cyano-3,5-dihydroxyhexanoate when no pre-stir is used, triethylborane affords a 5 to 10:1 (cis:trans) mixture. When diethylmethoxyborane is used in place of triethylborane, a 5 to 10:1 (cis:trans) mixture is obtained. When a combination of 10% by weight of diethylmethoxyborane and 90% by weight of triethylborane is used, typically a greater than 30:1 (cis:trans) mixture is obtained. This synergistic effect of combining a dialkylalkoxyborane and a trialkylborane could not have been predicted based on the use of either reagent alone or literature precedent.

The following examples are illustrative to show the present processes and to show the usefulness in the preparation of (4R-Cis) 1,1-dimethylethyl-6-cyanomethyl-2,2-dimethyl-1,3-dioxanc-4-acetate which is an intermediate prepared from a 1,3-diol of the present process that can be converted to atorvastatin ([R-(R*,R*)]-2-(4-fluorophenyl)-β,δ-dihydroxy-5-(1-methylethyl)-3-phenyl-4-[(phenylamino)carbonyl]-1H-pyrrole-1-heptanoic acid, calcium salt (2:1)) (crystalline Form I) which is useful as a hypolipidemic and hypocholesterolemic agent.

EXAMPLE 1 (4R-Cis) 1,1-Dimethylethyl-6-cyanomethyl-2,2-dimethyl-1,3-dioxane-4-acetate

Step (1): Preparation of 5R 1,1-Dimethylethyl 6-cyano-5-hydroxy-3-oxohexanoate

To a vessel containing 265 kg of 16.8% n-butyllithium is added a mixture of 80 kg of diisopropylamine in 80 L of tetrahydrofuran maintaining the temperature at less than 20° C. The solution is cooled to −55° C., and 85 kg of tert-butyl acetate is added maintaining the temperature at −50°±5° C. A solution of 25 kg of R 4-cyano-3-hydroxybutanoic acid ethyl ester in 55 L of tetrahydrofuran is then added, and the temperature is allowed to warm to −20° C. for at least 20 minutes. The solution is then quenched by transferring to aqueous hydrochloric acid. The organic layer is separated and the aqueous layer re-cxtracted with ethyl acetate. The combined organic layers are concentrated by vacuum distillation to afford crude 5R 1,1-dimethylethyl 6-cyano-5-hydroxy-3-oxohexanoate.

Step (2): Preparation of [R-(R*,R*)]-1,1-Dimethylethyl 6-cyano-3,5-dihydroxyhexanoate

Method A: Using Triethylborane

Step (a):

Crude 5R 1,1-dimethylethyl 6-cyano-5-hydroxy-3-oxohexanoate (about 150 moles) from Step (1) is dissolved in 325 L of tetrahydrofuran containing about 20 kg of triethylborane, stirred for about 2 hours at room temperature, cooled to ±75° C.±20° C. and diluted with 25 L of methanol and 8 kg of acetic acid. Sodium borohydride (8 kg) as a solution in methanol and aqueous sodium hydroxide is added slowly. After the addition, the reaction mixture is warmed to 0° C.±25° C. The reaction mixture is optionally quenched by the addition of 3 kcg of acetic acid and 10 L of methanol and concentrated by vacuum distillation, saving the distillate. The residue is dissolved in methanol and acetic acid, optionally diluted with water, and concentrated by vacuum distillation, keeping this distillate separate from the first one. The residue is dissolved in methanol and concentrated by vacuum distillation. The residue is dissolved in a mixture of water and ethyl acetate, and the aqueous layer separated. The organic layer is concentrated by vacuum distillation. The residue is dissolved in methanol and acetic acid and concentrated by vacuum distillation. The residue is dissolved in ethyl acetate and concentrated by vacuum distillation affording crude [R-(R*,R*)]-1,1-dimethylethyl 6-cyano-3,5-dihydroxyhexanoate.

The cis:trans ratio was about 30:1 as measured after conversion to (4R cis and trans) 1,1-dimethylethyl 6-cyanomethyl-2,2-dimethyl-1,3-dioxane-4-acetate according to the procedure described herein in Step 3.

Step (b): Reusing recovered triethylborane

Crude 5R 1,1-dimethylethyl 6-cyano-5-hydroxy-3-oxohexanoate (about 150 moles) from Step (1) is dissolved in the first distillate from Step (a) along with 50 L of tetrahydrofuran, cooled to −75° C.±20° C., and optionally diluted with 25 L of methanol, and 10 kg of acetic acid. Sodium borohydride (8 kg) as a solution in methanol and aqueous sodium hydroxide is added slowly. After the addition, the reaction mixture is warmed to 0° C.±25° C. The reaction mixture is quenched by the addition of 10 kg of acetic acid and 20 L of methanol and concentrated by vacuum distillation. The residue is dissolved in methanol and acetic acid, optionally diluted with water, and concentrated by vacuum distillation. The residue is dissolved in methanol and concentrated by vacuum distillation. The residue is dissolved in a mixture of water and ethyl acetate, and the aqueous layer separated. The organic layer is concentrated by vacuum distillation. The residue is dissolved in methanol and acetic acid and concentrated by vacuum distillation. The residue is dissolved in ethyl acetate and concentrated by vacuum distillation affording crude [R-(R*,R*)]-1,1-dimethylethyl 6-cyano-3,5-dihydroxyhexanoate.

The cis:trans ratio was about 40:1 as measured after conversion to (4R cis and trans) 1,1-dimethylethyl 6-cyanomethyl-2,2-dimethyl-1,3-dioxane-4-acetate according to the procedure described herein in Step 3.

Method B: Using Triethylborane

Step (a):

Crude 5R 1,1-dimethylethyl 6-cyano-5-hydroxy-3-oxohexanoate (about 130 mmoles) from Step (1) is dissolved in 100 mL 1 M triethylborane in THF and 65 mL tetrahydrofuran, stirred for about 2 hours at room temperature, then cooled to −75° C.±20° C. and diluted with 25 mL methanol. Sodium borohydride (6 g) as a solution in triglyme (75 mL) is added slowly. After the addition, the reaction mixture is warmed to 20° C. to 25° C. The reaction mixture is quenched by the addition of 20 mL methanol and 8 g acetic acid and concentrated by vacuum distillation—saving the distillate. The residue is diluted with 100 mL water and 200 mL ethyl acetate, agitated, and the phases separated. The organic layer is concentrated by vacuum distillation—keeping this distillate separate from the first one. The residue is dissolved in 200 mL methanol and 10 mL acetic acid and concentrated by vacuum distillation. The residue is dissolved in 200 mL methanol and concentrated by vacuum distillation. The residue is dissolved in 200 mL methanol and concentrated by vacuum distillation. The residue is dissolved in ethyl acetate and concentrated by vacuum distillation resulting in crude [R-(R*,R*)]-1,1-dimethylethyl 6-cyano-3,5-dihydroxyhexanoatc.

The cis:trans ratio was about 20:1 as measured after conversion to (4R cis and trans) 1,1-dimethylethyl 6-cyanomethyl-2,2-dimethyl-1,3-dioxane-4-acetate according to the procedure described herein in Step 3.

Step (b): Reusing Recovered Triethylborane

Crude 5R 1,1-dimethylethyl 6-cyano-5-hydroxy-3-oxohexanoate (about 130 mmoles) from Step (1) is dissolved in the first distillate from the above Step (a) mixture and cooled to −75° C.±20° C. Sodium borohydride (6 g) as a solution in 75 mL triglyme is added slowly. After the addition, the reaction mixture is warmed to 25° C. The reaction mixture is quenched by the addition of 8 g acetic acid (and optionally 20 mL methanol) and concentrated by vacuum distillation—saving the distillate. The residue is dissolved in a mixture of water (100 mL) and ethyl acetate (200 mL), the layers separated, and the organic layer is concentrated by vacuum distillation. The residue is dissolved with 200 mL methanol and 10 mL acetic acid and concentrated by vacuum distillation. The residue is dissolved with 200 mL methanol and concentrated by vacuum distillation. The residue is dissolved with 200 mL methanol and concentrated by vacuum distillation. The residue is dissolved in ethyl acetate and concentrated by vacuum distillation resulting in crude R-(R*,R*)]-1,1-dimethylethyl 6-cyano-3,5-dihydroxyhexanoate.

The cis:trans ratio was about 30:1 as measured after conversion to (4R cis and trans) 1,1-dimethylethyl 6-cyanomethyl-2,2-dimethyl-1,3-dioxane-4-acetate according to the procedure described herein in Step 3.

Step (c): Reusing Recovered Triethylborane

Following the procedure of the previous Step (b) crude 5R-1,1-dimethylethyl 6-cyano-5-hydroxy-3-oxohexanoate (about 130 mmoles) from Step (1) is reacted with recovered triethylborane from Step (b) to afford crude [R-(R*,R*)]-1,1-dimethylethyl 6-cyano-3,5-dihydroxyhexanoate.

The cis:trans ratio was about 30:1 as measured after conversion to (4R cis and trans) 1,1-dimethylethyl 6-cyanomethyl-2,2-dimethyl-1,3-dioxane-4-acetate according to the procedure described herein in Step 3.

Method C: Using Diethylmethoxyborane

Step (a):

Crude 5R 1,1-dimethylethyl 6-cyano-5-hydroxy-3-oxohexanoate (about 150 mmoles) is dissolved by adding 22 mL diethylmethoxyborane and 200 mL tetrahydrofuran. The solution is stirred for about 2 hours at room temperature, then cooled to −70° C. to −75° C., and further diluted with 25 mL methanol. Sodium borohydride (6 g) as a solution in triglyme (75 mL) is added slowly at between −65° C. to −75° C. After the addition, the reaction mixture is warmed to 15° C. to 25° C., quenched by the addition of acetic acid and concentrated by vacuum distillation—keeping this distillate. The residue is diluted with methanol and concentrated by vacuum distillation—keeping this distillate and all subsequent ones separate from the first one. The residue is dissolved in a mixture of water and ethyl acetate, the layers separated, and the organic layer is concentrated by vacuum distillation. The residue is dissolved in methanol and acetic acid and concentrated by vacuum distillation. The residue is dissolved in ethyl acetate and concentrated by vacuum distillation resulting in crude [R(R*,R*)]-1,1-dimethylethyl 6-cyano-3,5-dihydroxyhexanoate.

The cis:trans ratio was about 35:1 as measured after conversion to (4R cis and trans) 1,1-dimethylethyl 6-cyanomethyl-2,2-dimethyl-1,3-dioxane-4-acetate according to the procedure described herein in Step 3.

Step (b): Reusing Recovered Diethylmethoxyborane

Crude 5R 1,1-dimethylethyl 6-cyano-5-hydroxy-3-oxohexanoate (about 150 mmoles) is dissolved in the first distillate from the above mixture, allowed to stir at room temperature for about 2 hours, and cooled to about −70° C. Sodium borohydride (6 g) as a solution in 75 mL triglyme is added slowly at between −65° C. to −75° C. After the addition, the reaction mixture is warmed to 15° C. to 25° C., quenched by the addition of acetic acid and concentrated by vacuum distillation. The residue is diluted with methanol and concentrated by vacuum distillation. The residue is dissolved in a mixture of water and ethyl acetate, the layers separated, and the organic layer is concentrated by vacuum distillation. The residue is dissolved with methanol and acetic acid and concentrated by vacuum distillation. The residue is dissolved in ethyl acetate and concentrated by vacuum distillation resulting in crude [R(R*,R*)]-1,1-dimethylethyl 6-cyano-3,5-dihydroxyhexanoate.

The cis:trans ratio was about 25:1 as measured after conversion to (4R cis and trans) 1,1-dimethylethyl 6-cyanomethyl-2,2-dimethyl-1,3-dioxane-4-acetate according to the procedure described herein in Step 3.

Method D: Using a Mixture of Diethylmethoxyborane and Triethylborane

Step (a):

Crude SR 1,1-Dimethylethyl 6-cyano-5-hydroxy-3-oxohexanoate (about 150 mmoles) is dissolved in 170 mL tetrahydrofuran. The solution is cooled to −70° C. to 75° C., and further diluted with 115 mL of 14% triethylborane in tetrahydrofuran, 4 mL diethylmethoxyborane, 45 mL methanol, and 8 mL acetic acid. Sodium borohydride (7 g) as a solution in methanol (65 mL) containing 50% aqueous sodium hydroxide (3.2 g) is added slowly at between −70° C. to −75° C. After the addition, the reaction mixture is warmed to 15° C. to 25° C., quenched by the addition of acetic acid, and concentrated by vacuum distillation—keeping the distillate. The residue is diluted with methanol and concentrated by vacuum distillation—keeping this distillate and all subsequent ones separate from the first one. The residue is dissolved in a mixture of water and ethyl acetate, the layers separated, and the organic layer is concentrated by vacuum distillation. The residue is dissolved in methanol and acetic acid and concentrated by vacuum distillation. The residue is dissolved in ethyl acetate and concentrated by vacuum distillation resulting in crude [R-(R*,R*)-1,1-dimethylethyl 6-cyano-3,5-dihydroxyhexanoate.

The cis:trans ratio was >50:1 as measured after conversion to (4R cis and trans) 1,1-dimethylethyl 6-cyanomethyl-2,2-dimethyl-1,3-dioxane-4-acetate according to the procedure described herein in Step 3.

Step (b): Reusing Recovered Triethylborane and Diethylmethoxyborane Mixture

Following the procedure in Step (2)(h) as described in Method A affords [R-(R*,R*)-1,1-dimethylethyl 6-cyano-3,5-dihydroxyhexanoate.

Step (3): Preparation of (4R cis) 1,1-Dimethylethyl 6-cyanomethyl-2,2-dimethyl-1,3-dioxane-4-acetate

Crude [R-(R*,R*)]-1,1-dimethylethyl 6-cyano-3,5-dihydroxyhexanoate (about 150 moles) from Step (2) is diluted with 100 kg of 2,2-dimethoxypropane and acidified with about 1 L of methanesulfonic acid. The reaction is quenched by the addition of aqueous sodium bicarbonate solution and concentrated by vacuum distillation. The residue is diluted with 150 L of hexane, and the layers separated. The organic layer is washed with aqueous sodium bicarbonate solution and cooled to 0° C.±10° C. to crystallize. The product is collected by filtration and washed with cooled hexane, then dried affording 28.5 kg of (4R cis) 1,1-dimethylethyl 6-cyanomethyl-2,2-dimethyl-1,3-dioxane-4-acetate.

EXAMPLE 2 [R-(R*,R*)]-2-(4-Fluorophenyl)-β,δ-Dihydroxy-5-(1-methylethyl)-3-phenyl-4-[(phenylamino)carbonyl]-1H-pyrrole-1-heptanoic Acid, Calcium Salt (2:1) (crystalline Form I)

Step (1): Preparation of (4R-cis)-1,1-dimethylethyl 6-(2-aminoethyl)-2,2-dimethyl-1,3-dioxane-4-acetate

(4R-cis)1,1-Dimethylethyl 6-cyanomethyl-2,2-dimethyl-1,3-dioxane-4-acetate (Example 1) is converted to the title compound using the methodology disclosed at Column 49, Lines 16-43 of U.S. Pat No. 5,003,080.

Step (2): Preparation of (4R-cis)-1,1-Dimethylethyl 6-[2[2-(fluorophenyl)-5-(1-methylethyl)-3-phenyl-4-[(phenylamino)carbonyl]-1H-pyrrol-1-yl]ethyl]-2,2-dimethyl-1,3-dioxane-4-acetate

(4R-cis)-1,1-Dimethylethyl 6-(2-aminoethyl)-2,2-dimethyl-1,3-dioxane-4-acetate is converted to the title compound using the methodology disclosed at Column 49, Lines 43-60 of the U.S. Pat No. 5,003,080.

Step (3): Preparation of (2R-trans)-5-(4-fluorophenyl)-2-(1-methylethyl)-N, 4-diphenyl-1-[2-(tetrahydro-4-hydroxy-6-oxo-2H-pyran-2-yl)ethyl]-1H-pyrrole-3-carboxamide

(4R-cis)-1,1-Dimethyl 6-(2-aminoethyl)-2,2-dimethyl-1,3-dioxane-4-acetate is converted to the title compound using the methodology disclosed at Column 50, Lines 4-30 of U.S. Pat No. 5,003,080.

Step (4): Preparation of [R-(R*,R*)]-2-(4-fluorophenyl)-β,δ-dihydroxy-5-(1-methylethyl)-3-phenyl-4-[(phenylamino)carbonyl]-1-1H-pyrrole-1-heptanoic Acid, Calcium Salt (2:1) (crystalline Form I)

(2R-trans)-5-(4-Fluorophenyl)-2-(1-methylethyl)-N,4-diphenyl-1-[2-(tetrahydro-4-hydroxy-6-oxo-2H-pyran-2-yl)ethyl]-1H-pyrrole-3-carboxamide is converted to the title compound using the methodology disclosed in copending U.S. patent application Ser. No. 08/945,812. 

What is claimed is:
 1. A process for the preparation of a compound of Formula I

wherein R is alkyl, NC—CH₂—, or PG—O—CH₂— wherein PG is a protecting group; and R¹ is alkyl, or —CH₂—CO₂R⁶ wherein R⁶ is alkyl; which comprises: Step (a) treating a compound of Formula II

or a compound of Formula III

wherein R and R¹ are as defined above with a trialkylborane or dialkylalkoxyborane or a mixture of a trialkylborane and a dialkylalkoxyborane in a solvent; Step (b) adding an alkali metal hydride at about −110° C. to about −50° C.; Step (c) concentrating the reaction by distillation to afford a compound of Formula I and a distillate containing alkylborane species; and Step (d) treating additional compound of Formula II or III with the distillate from Step (c) containing recovered alkylborane species, and repeating Steps (b) and (c) as desired to afford additional compound of Formula I.
 2. The process according to claim 1 wherein the trialkylborane in Step (a) is triethylborane.
 3. The process according to claim 1 wherein the dialkylalkoxyborane in Step (a) is diethylmethoxyborane.
 4. The process according to claim 1 wherein the solvent in Step (a) is selected from the group consisting of: tetrahydrofuran; methanol; and a mixture of tetrahydrofuran and methanol.
 5. The process according to claim 1 wherein the distillation in Step (c) is a vacuum distillation.
 6. The process according to claim 1 wherein PG is benzyl.
 7. The process according to claim 1 wherein the alkali metal hydride is sodium borohydride.
 8. The process according to claim 1 for the preparation of [R-(R*,R*)]-1,1-dimethylethyl 6-cyano-3 5-dihydroxyhexanoate.
 9. A process for the preparation of a compound of Formula I

wherein R is alkyl, NC—CH₂—, or PG—O—CH₂— wherein PG is a protecting group; and R¹ is alkyl, or —CH₂—CO₂R⁶ wherein R⁶ is alkyl; which comprises: Step (a) treating a compound of Formula II

or a compound of Formula III

wherein R and R¹ are as defined above with a synergistic combination of a trialkylborane and a dialkylalkoxyborane in a solvent; and Step (b) adding an alkali metal hydride at about −110° C. to about −50° C. to afford a compound of Formula I.
 10. The process according to claim 9 wherein the trialkylborane in Step (a) is triethylborane.
 11. The process according to claim 9 wherein the dialkylalkoxyborane in Step (a) is diethylmethoxyborane.
 12. The process according to claim 9 wherein the solvent Step (a) is selected from the group consisting of: tetrahydrofuran; methanol; and a mixture of tetrahydrofuran and methanol.
 13. The process according to claim 9 wherein PG is benzyl.
 14. The process according to claim 9 wherein the alkali metal hydride is sodium borohydride.
 15. The process according to claim 9 for the preparation of [R-(R*,R*)]-1,1-dimethylethyl 6-cyano-3,5-dihydroxyhexanoate. 